When people read a book or newspaper, they usually put the printing on a desk having a surface parallel to a horizontal plane. FIG. 1 shows that a person views obliquely a printing put on a horizontal plane. When the person views the printing in an oblique direction as shown in FIG. 1, an angle (θ1) of a direction viewing a top portion of the printing from a viewpoint with respect to a direction perpendicular to the printing is larger than an angle (θ2) of a direction viewing a bottom portion of the printing, and a distance from characters in the top portion of the printing to the viewpoint of the reader is relatively farther than that from characters in the bottom portion of the printing to the viewpoint of the reader. According to the perspective, even though they are the characters of same size (d) in fact, the size of the characters perceived by human eyes looks smaller and smaller as going from the bottom portion to the top portion (that is, S1>S2).
In order to get rid of this distortion of the character size, the book or newspaper may be put up so that the printing surface is perpendicular to the direction of viewing, but it is going to be very uncomfortable to keep reading the book or newspaper for an extended time period.